1. Technical Field
The invention relates to vibrating beam force-frequency transducers which make it possible to obtain force measuring apparatuses having a small mass and volume. It relates more precisely to transducers of the type comprising an elongate strip of piezo-electric material embedded at both its ends in elements applying a longitudinal force to the strip and split in its middle portion so as to form two lateral beams carrying electrodes for vibrating the beams in the plane of the major faces of the strip and for measuring the vibration frequency.
With such a transducer, the tractive force applied to the ends of the strip may be measured. It finds a particularly important, although not exclusive, application in non-servoed pendulous accelerometers comprising a pendular seismic mass or proof mass connected to a base by an articulation allowing it to move in a sense direction perpendicular to the articulation axis. The ends of the transducer are then secured to the base and to the mass.
2. Prior Art
Numerous transducers of the above-defined type are already known. Examples are given in U.S. Pat. No. 3,238,789 (Erdley) and French 2,454,614 (U.S. Department of Energy). According to the latter document, it would be essential that the distance separating the two beams be very small and form a slit whose width is less than the thickness of the beam. The reason alleged for this condition is that the slit must be narrow for the forces applied to the two beams to be equal and for the Q factor to be high and to give an acceptable stability.
Even if transducers of the above-defined type may effectively be produced having a very small mass and volume and an acceptable sensitivity, they have drawbacks. The main one is that their force-frequency response varies as a function of the temperature. For a known type transducer which may be considered as typical, the scale factor (frequency variation when under an acceleration equal to g=9.81 ms.sup.-2) ranges from 0.5 to a few Herz. Now, for an operating temperature of 70.degree. C., which value is usual in numerous inertial equipments, the parabolic frequency variation is 7.5.10.sup.-2 Hz per .degree.C.: a difference as small as 0.1.degree. C. between the two strips of the sensor results in an error of 7.5.10.sup.-2 g on the acceleration, which is high as compared with the sensitivity.